read and extract informations from file c++

Question

I am trying to find a solution for the error that the c++ compiler (codeblocks) keeps showing to me,I searched for answers on the net but none of them seemed to be helpful. I have a txt file in which there are numbers,each of them is written in a line. What i want is to read each number(line by line)and store it in a variable after converting it from string to float. This my code

#include <fstream>
#include <sstream>
#include <string>
#include <iostream>

float z;   
int j=0;
stringstream s; 
const ifstream fich1; 
fich1.open("test.txt",ios::in);     
if(!fich1)
{ 
    cerr<<"could not open the file";
    EXIT_FAILURE;
};
else const string ligne1;
while((!fich1.eof())
{ 
    if(j!=i)
    {
        j++;
        getline(fich1,l1); // the first error is: no matching function for call to ‘getline(const ifstream&, const string&)
    else if(j == i)
    {   
        fich1>>s; // the second error is: ambiguous overload for ‘operator>>’ (operand types are ‘const ifstream {aka const std::basic_ifstream<char>}’ and ‘std::stringstream {aka std::basic_stringstream<char>}’)
        z=(float)s; // I couldn't find a clear answer to convert from string to float
    }
}

if anyone wants to ask any question about the code to make it clearer may ask it please,I am waiting for your answers as well as your questions :)

Answer 1

After the edit, I am able to read some code, but still I am suggesting the example I have below, since I am see scary things, like an EOF inside a loop!

Oh also, if you have C++11 support, then you could use std::stof.

---

You could try this (since your post is not readable), which reads a file line by line and stores every float number in a vector:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <cstdlib>

int main() {
  std::vector<float> v;
  std::string rec_file = "test.txt";
  std::string line;
  std::ifstream myfile(rec_file.c_str());
  if(myfile.is_open()) {
    while(std::getline(myfile,line)) {
      if(!line.empty())
        v.push_back(atof(line.c_str()));
    }
    myfile.close();
  } else {
      std::cerr << "Unable to open file";
  }
  for(unsigned int i = 0; i < v.size(); ++i)
    std::cout << v[i] << std::endl;
  return 0;
}

with the test.txt to be:

1.2
2.3
3.4
4.5
5.6
6.7

and the output:

1.2
2.3
3.4
4.5
5.6
6.7

Answer 2

Your ifstream shouldn't be constant, as getline alters the ifstream.

To convert from char array to float use atof(chararray)

To convert from string to float you could use atof(string.cstr())

Answer 3

You cannot use const here. But that's not your only problem.

#include <fstream>
#include <sstream>
#include <string>
#include <iostream>

Looks good.

float z;   
int j=0;
stringstream s; 

There should be some int main() { here, because that is the function called by the runtime when a C++ executable starts. This isn't Bash or Perl where execution simply picks up at the first statement.

And you either need to write using namespace std; or prefix identifiers like stringstream as std::stringstream.

const ifstream fich1; 

This won't work. ifstream must not be declared const. (This is actually the explanation for the errors you encountered, but you made many more.)

An input file stream is an object with complex inner state, like where in the file you're currently at, error flags etc.; this inner state is changing so const ifstream simply won't work.

fich1.open("test.txt",ios::in);     

This can't work either due to const, just like all the other operations on fich1 you're doing further down.

if(!fich1)
      { cerr<<"could not open the file";
      EXIT_FAILURE;
     };

EXIT_FAILURE is a symbolic constant for a return value. As-is, you could just as well have written 0;. If you expected this to end your program, you're wrong.

      ;

The semicolon ends the if.

      else

Since the if has ended, this is a syntax error.

         const string ligne1;

If you declare a string const, you cannot assign to it. Besides, even if the else were correct, that semicolon would have ended the else, because you didn't add braces (as you should always do, even with one-line blocks, because it's so easy to make mistakes otherwise).

The way the code continues, I very much doubt this was your intention. (Some discipline with the indenting makes this kind of mistakes really easy to spot.)

         while((!fich1.eof())

How to read until EOF from cin.

         { if(j!=i)
             {j++;

Do some proper indenting, will you? Besides, no i has been declared at this point.

         getline(fich1,l1);

I assume l1 is the string you declared earlier as ligne1. (Since that happened inside the else, ligne1 is no longer in scope, but I'll let that one pass since you obviously intended the while to be inside the else block.)

Anyway, this can't work because both your ifstream and your string are constant, i.e. cannot be changed. This does not make sense, and thus getline() was not defined for const parameters.

             else if(j == i)

Since you didn't close the brace of the if statement above, this is a syntax error as well. Again, proper indenting discipline would have made this immediately obvious.

             {   fich1>>s;

I very much doubt there exists an operator>>() for const ifstream, so I am not surprised you get an error here.

                 z=(float)s;

You're trying to cast (C style, too...) a stringstream object to a float number? What do you expect the result might be? It will definitely not be the 3.14 or whatever you've written into test.txt...

               }
}

Lost track of your braces. This code is FUBAR, I didn't even check if it makes sense semantically once all the errors are fixed, and I suspect you're pulling some elaborate troll prank here. If that isn't the case, I suggest you take another good look at whatever textbook you're using to learn C++, because you got a good many things very wrong.

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Purely style-related advice: fich1, ligne1... don't use localized (non-English) identifiers. It just adds another problem when communicating about your code. (And this is coming from a non-native English speaker.)